Geologists have long thought that Earths first 500 million years were as
hot as Hades, dubbing this time frame the Hadean. The high temperatures would
have prevented liquid water from condensing on the surface. But new findings on
zircon grains, Earths oldest known terrestrial materials, suggest that the
Hadean might have hosted liquid water.

Recovered from the
metamorphosed sediments of the Jack Hills in western Australia, the zircon grains
are dated to be more than 4 billion years old and are the only geological evidence
available to provide insight into the first 500 million years of Earths
history.

This period is also considered the dark ages of the Earth because there
are no known rocks that have been preserved, says John Valley from the University
of Wisconsin-Madison, lead investigator in the Jack Hills zircon research. Presenting
at the Geological Society of Americas annual meeting in Denver last October,
Valley and his colleagues suggest geochemical clues in the zircons provide the
surprising news that early Earth was cool enough for liquid water to exist.

Eranondoo Hill in the Jack Hills of
western Australia contains zircon grains as old as 4.4 billion years. A team led
by John Valley of the University of Wisconsin-Madison (left) found surprising
clues in the grains. Also pictured are Aaron Cavosie of the University of Wisconsin-Madison
and Simon Wilde of Curtin University. Photo by David Valley.

In the late 1990s, Valley and his co-workers realized that zircons accurately
preserve their original oxygen isotope values, and they decided to document zircons
through all of geological time. This realization prompted their discovery of the
oldest Jack Hills zircon, a 4.4-billion-year-old detrital grain. The Hadean hypothesis
holds that Earth had not yet developed any source materials other than molten
magma generated from the interior or from a meteorite bombardment. When the team
first placed the grain on the ion microprobe, they expected it to have oxygen
isotope ratios of a zircon crystallized in a rock that would in turn have mantle
geochemical signatures. But the values of the grains oxygen isotopes were
much higher than they expected.

Rocks that have zircons with higher oxygen isotope values indicate a source
that has interacted with water at low temperatures, says Aaron Cavosie,
also from the University of Wisconsin-Madison. The new isotope values have two
implications, Cavosie says. First, they suggest that water existed as early as
4.4 billion years ago. Because water is a prerequisite for life, this research
potentially pushes back the time that life could have originated. Prior to this
research, the first known rock evidence for water on Earth was from the Isua metasediments
in Greenland, which date to 3.8 billion years ago (Geotimes, July 2002).

Second, the data suggest that the zircons formed in a source rock that was contaminated
with material that had interacted with water at low temperatures  that is,
rock close to the surface. The trace element geochemistry and quartz inclusions
of the zircons corroborate the oxygen isotope data, and point to an evolved rock,
much like a granite, indicating that continental crust probably existed on early
Earth.

These are the first real data that suggest that there was supracrustal material
that early on Earth, says co-author William Peck of Colgate University.
In addition, the possibility of liquid water  and perhaps oceans 
this early in the planets formation is really exciting. It has implications
for when catastrophic bombardment of the Earth by meteorites likely waned.
Meteorite bombardments are heat sources, so the idea that early Earth was cool
enough for water raises questions regarding the timing and intensity of bombardments.

Alex Halliday, a geochemist from the Institute of Isotope Geology and Mineral
Resources in Zurich, suggests caution about the findings. They are not being
outrageous or even factually incorrect. It is just that there are inevitable,
fundamental assumptions involved in scaling up a portion of a single grain of
zircon to the existence of continents and oceans.

Valley and his colleagues agree. The cool early Earth hypothesis is controversial,
and we are working very hard to test it, Valley says. Whereas in 2001 they
had only one zircon older than 4.3 billion years, they now have several, which
they are using to test their interpretations.

The Jack Hills zircons are proving to be a treasure trove for understanding early
Earth. The most important aspect is that we now have an archive to work
on, albeit limited, Halliday says. The past 10 years have seen major
revisions in our understanding of the earliest Earth, and this field has been
viewed as one of the most interesting and rewarding today in terms of significant
discoveries.